The present invention relates to an image recording apparatus for recording an image, especially a still image obtained by a solid-state image pickup device.
Recently, image recording apparatuses, called "electronic cameras", have been made commercially available, in lieu of conventional cameras using silver chloride films, to record a still image as an image signal. A solid-state image pickup device serving as an image pickup camera is used in the image recording apparatus of this type, to satisfy the need for portability.
In conventional photography for general and scientific purposes, exposure times often exceed 10 seconds. The exposure time of the conventional camera corresponds to a light-receiving time interval of the solid-state image pickup device of the electronic camera.
The solid-state image pickup device stores a signal charge produced by incident light energy during the light-receiving time interval. In general, the solid-state image pickup device produces not only a signal current corresponding to incident light energy, but also a dark current due to heat or the like. When the solid-state image pickup device is exposed to light for a long period of time, a dark current charge which is greater than the signal charge is often stored. This dark current charge has the same polarity as that of the signal charge. It is very difficult to remove the dark current component from the signal component after the light-receiving period. As a result, long exposure cannot be generally performed in the solid-state image pickup device, since the S/N ratio is lowered and the image quality is degraded. Furthermore, the solid-state image pickup device can store only a small amount of charge therein. When a large amount of dark current charge is stored in the solid-state image pickup device by long exposure, the dynamic range of the camera is narrowed, thus degrading image quality. In this manner, long exposure cannot be performed in the electronic camera.